The present invention relates to the field of transcutaneous gas measuring apparatus for non-invasive measurement of blood gases which are transcutaneous through the skin of a living body. More specifically, the invention concerns an apparatus and method for determining the amount of a gas which is transcutaneous through the skin at a predetermined reference temperature while permitting variations in temperature from the reference temperature during the measurement and, hence, without need to heat or otherwise control the temperature of the skin or measuring apparatus during the measurement.
It is known in the medical art of non-invasive blood gas content monitoring and measurement to apply to the surface of the skin of the person whose blood gas content is to be monitored and measured, a probe having a barrier permeable to the gas to be measured. The gas to be measured is permitted to diffuse through the barrier which is normally a membrane having specific diffusion properties and into a solution stored above the membrane in which the gas is soluble. An electrode assembly in contact with the solution is used to measure the effect of the dissolved gas on the electrical properties of the solution to provide a quantitative indication of the amount of gas emitted from the skin.
In the case of transcutaneous carbon dioxide measurement, a sensor is used to measure the pressure of the carbon dioxide transcutaneous through the skin at a region of the skin to which the membrane assembly of the electrode is applied. The anode, which is referred to as the reference electrode, and cathode, which is referred to as the pH electrode of the transcutaneous carbon dioxide sensor, are in contact with an electrolytic solution in which the carbon dioxide transcutaneous through the permeable membrane is dissolved. As a result of the dissolving of the carbon dioxide in the electrolytic solution, a voltage is induced between the reference and pH electrodes which is proportional to the logarithm of the transcutaneous carbon dioxide pressure or tension at the skin surface. Hence, the induced voltage can be processed by suitable circuitry and then displayed on a volt meter to give a direct reading of transcutaneous carbon dioxide pressure or tension.
It is known in the art of medicine that transcutaneous carbon dioxide pressure varies significantly with changes in temperature. In order to make useful diagnoses of a patient's condition based on transcutaneous carbon dioxide pressure, it has been found necessary to obtain all transcutaneous carbon dioxide pressure readings at a standard predetermined reference temperature and to compare such measurements with standards of what are normal and abnormal transcutaneous carbon dioxide pressures developed for the same reference temperature.
Prior art transcutaneous carbon dioxide measurement devices have dealt with the problem of temperature variation during transcutaneous carbon dioxide measurements by employing means to stabilize the temperatures of the skin and engaging measurement apparatus. Some transcutaneous blood gas measuring systems employ probes which include electrical heaters that are thermostatically controlled to maintain the probe and adjacent skin temperatures as close as possible to the reference level. U.S. Pat. No. 3,659,586 to Johns et al for a Percutaneous Carbon Dioxide Sensor and Process for Measuring Pulmonary Efficiency discloses a transcutaneous carbon dioxide measuring probe which includes a lead baseplate to serve as a thermal sink for minimizing temperature changes.
The approaches taken in the prior art to prevent temperature variations are only partially effective as constantly changing environmental conditions as well as metabolic conditions of the patient continuously cause temperature variations and fluctuations all of which degrade the accuracy of transcutaneous carbon dioxide pressure measurements. Moreover, the necessity for adding temperature stabilization devices to the gas measuring probes, such as heaters and thermal sinks, increases the cost and complexity of the probes, and can result in discomfort to the patient.